1. Field of the Invention
The invention is concerned with improved ultra-filtration membranes in which the flux rate is improved by the presence of a finely-divided, inert, water-insoluble and impervious particulate matter wholly incorporated in the membrane, particularly in the region adjacent the interface of the membrane with its support surface. The membranes are suitable for concentrating solutions and dispersions of comparatively large molecules, for example protein in whey and milk.
2. The Prior Art
Membrane filtration is now well established as a means for effecting the separation of components in a liquid mixture, usually in an aqueous vehicle, which differ in their effective molecular sizes. The membranes are in the form of a thin, coherent film made from polymeric material, usually man-made and either of synthetic polymers or of a modified cellulose base. The membranes are classified according to the molecular dimensions of the pores through which permeation takes place. These dimensions in turn determine the pressure which must be applied to the solutions undergoing filtration. Membranes with small pores may permit the passage of water but not of salt ions. Substantial pressures, of several hundred pounds per sq. inch, may then be necessary to effect the separation of water from the solution by means of such membranes, to counter the considerable osmotic pressure which is generated in consequence, across the membrane before water can permeate through it. In these reverse osmosis processes, the hyperfiltration membranes are commonly made by casting a film from a dope of the polymer dissolved in a volatile membrane and allowing the solvent to evaporate. The membrane is then almost invariably cured by heating.
Reverse osmosis processes are widely used for obtaining potable water from brackish water and especially from sea water, and for concentrating inorganic sewage. Hyperfiltration membranes have also been used to effect a concentration of aqueous materials, for example fruit juices, whey, milk and other aqueous protein sources, but the flux rates through these membranes are unduly low and the pressures at which they must be used excessively high. Early attempts have been made to enlarge the pore size of these membranes by including within the casting dope a proportion of pore-enlarging material. An early description of hyperfiltration membranes is given by Loeb et al U.S. Pat. No. 3,133,132 and 3,133,137, the membrane being cast as already described, from a dope comprising a volatile solvent in which the polymer is dissolved, together with pore-enlarging perchlorate salt. The effect of these additives in hyperfiltration membranes has however been shown to be unsatisfactory, leading to tenderisation of the membrane material and tending to form flaws in the membrane.
More recently, membranes have been developed which are permeable to salt solutions but reject larger molecules that themselves generate negligible osmotic pressure. Such membranes may therefore be used to concentrate larger molecules and being permeable to salts in addition to water, operate at substantially lower pressures than hyperfiltration membranes. These ultrafiltration membranes are prepared similarly from a casting dope of the polymer, dissolved however in a non-volatile solvent, for example dimethyl formamide and dimethyl sulphoxide. A description of ultrafiltration membranes and their properties appears in the Saline Water Conversion Report, 1968, page 191 and in German published Pat. No. 2,207,656. The present invention is concerned with improving the flux rate of the ultrafiltration membranes still further.
This invention relates to membrane filtration processes and to improved semi-permeable membranes for use in such processes.
Semi-permeable membranes used in membrane filtration processes enable the separation to be effected of material down to molecular dimensions, usually from aqueous systems. According to the selectivity of the membranes, otherwise expressed as the rejection characteristics, they find widespread application for example in desalinating brine, purifying effluent and concentrating milk protein, particularly in whey.
In hyperfiltration processes in which small solute molecules of molecular weight less than about 100 can be separated, membranes of fine pore size are employed in conjunction with filtration pressures of 1,000 psi or more which are necessary to overcome th considerable osmotic pressure generated by the small molecules. The selective rejection of much larger molecules, eg proteins, of molecular weight generally over 1,000, is effected on the other hand by membranes of more open pore structure, in ultrafiltration processes in which osmotic pressure is negligible and in which therefore substantially lower filtration pressures are adequate, generally about 100 psi or even less.
The present invention provides a semi-permeable, ultrafiltration membrane, suitable for use in ultrafiltration processes, in which a minor amount of an inert, impervious, water-insoluble, preferably inorganic, solid material in the form of finely-divided non-colloidal particles is dispersed wholly within the membranes so as to swell or otherwise change the structure of the membrane, thereby increasing its flux rate.
It has been found that the membranes of the invention can exhibit up to 2-3 times the flux, at a given pressure and temperature of otherwise identical membranes without the added particles. On the other hand, the rejection characteristics of the membranes towards protein and other large organic molecules which can normally be separated by ultrafiltration remain substantially unaffected. The flux rate is the flow rate that can be treated by unit area of membrane, and is commonly expressed in gallons per 24 hours per ft.sup.2, either US or Imperial gallons.
The exact mechanism by which the inorganic material improves the membranes is not known. The filtration of proteins in milk or other aqueous systems is adversely affected by the build-up of proteins on the surface of the membrane, forming a second "filter" having a poor flux rate. Without wishing to diminish the scope of the invention described by any expression as to its mechanism, it is believed that filtration through the membranes of this invention is improved by charged groups carried by the particles, effective amounts of which thus repel milk proteins from the membrane surface. This leads to higher permeation rates of water and dissolved salts through the membane, by lining the surface of passages through the membrane skin to give these areas a negative charge and thereby allowing effusion of neutrally-charged molecules through the membrane while rejecting charged molecules such as protein. it should be emphasised that the membranes of the present invention being of open pore structure, exercise no selective filtration action on aqueous solutions of small solutes, eg brine solutions, capable of exerting a strong osmotic pressure and are thus distinguished from reverse osmosis membranes which do so. Their application in ultrafiltration processes lies in their selective rejection of comparatively large molecules, for example, proteins. The limits of their effectiveness for this purpose, that is, the minimum size of molecules which they are capable of rejecting, depends largely upon the effective pore size of the membrane and hence upon the conditions and materials of its preparation but also upon the conditions under which it is used, particularly the operating pressure, increased pressure often effectively decreasing pore size. This is particularly observed where, as in milk and whey concentration using ultrafiltration methods, a wide spectrum of solute molecular sizes is present, providing a build-up of the bigger rejected solute molecules on the membrane in a layer which itself exercises a filtration action in the smaller molecules to which the membrane itself is non-rejecting. Thus, lactose solutions may be found to be filtered unchanged through a membrane which will however at least partially reject the lactose in milk or whey in the presence of the protein molecules, and the degree of rejection may then be enhanced with increased pressure above that customarily adopted for ultrafiltration.
Semi-permeable membranes are generally cast from a solution, usually referred to as dope, of a film-forming polymer in an organic solvent, the membranes used in hyperfiltration processes being cast from volatile solvent, for example acetone. The ultrafiltration membranes of the present invention, however, are conveniently prepared from dope comprising non-volatile organic solvents having a boiling point substantially in excess of 100.degree. C. Suitable solvents include dimethyl formamide, dimethyl sulphoxide and triethyl phosphate. It is surprising that membranes cast from volatile solvents show no improvement when particles are incorporated but on the contrary often exhibit flaws and are then wholly unsuitable for use. An important feature of the present invention is in the preparation of cellulose acetate ultrafiltration membranes, particularly from solutions in dimethyl formamide. These membranes can be used at elevated temperatures up to approximately 80.degree. C., enabling ultrafiltraton processes to be carried out at temperatures at which, for example, milk or whey may be pasteurised.
The concentration of polymer in the dope is not critical. Solutions from about 5% to 50% and above may be used if desired, up to the limits of solubility of the polymer. Preferably, however, a solution of 10-30% by weight concentration is used. Very dilute solutions tend to form very fragile membranes, while those prepared from very concentrated solutions may be tough but are often slow in use.
The invention also provides a method of preparing improved ultrafiltration membranes in which a solution of film-forming polymeric material, for example a cellulose ether or ester, is dissolved in a non-volatile solvent and as inert, finely-divided inorganic water-insoluble material is added having a mean specific surface area of preferably at least 50 m.sup.2 /g, and distributed throughout the solution, a film is cast and the solvent is leached by contact with a miscible solvent in which the polymer is insoluble.
The particles should be small compared to the molecular size of the membrane material. Bigger particles exceeding the cellular dimensions tend to form gaps in the membrane. Only a comparatively small concentration is needed to provide effective cover for all the membrane interfaces with the liquid to be filtered. The particles should preferably constitute at least 1% of the membrane casting solution, preferably 1-4% for carbon particles and from 4-25% is particularly preferred for metal particles. These amounts are expressed in the specification by weight, as grammes per cc of solution, 1% therefore representing 1 gramme as additive per 100 cc of solution. Greater amounts, up to 50%, may enhance the membrane flux still further, bur some loss may then occur of selectivity, to give a lower rejection factor towards molecules of specified size. The concentration at which this occurs is dependent upon the nature of the casting dope, including the size and nature of the active material. With these greater quantities the membranes may then become selective only towards th bigger molecules such as bacteria, while passing even milk protein, or defects in the membrane may develop. However, as much as 50% may be acceptable of some material, eg metal particles, without loss of milk protein rejection. In general also a greater change is effected using dimethyl sulphoxide than dimethyl formamide as the casting solution.
Suitable material to be added to the membrane is the form of particles in accordance with the invention include lamp black, carbon black and soot. Other inorganic materials which may be used include iron, and ferrous alloys including steel, metals generally if these are stable, both elements and their alloys, particularly nickel, cobalt, aluminum and their oxides, silica, silicon, sulphur and alumina. The materials should be substantially insoluble in water and the casting solvent, and exert no hydrolytic, catalytic, oxidative, reductive or other chemical change likely to lead to deterioration of the casting solvent or membrane material. They should be impermeable and should not penetrate the membrane when this is formed. The particles should not form suspensions in water.
A wide range of particle sizes may be adopted, but the best size range may vary from one material to another. Thus, for carbon particles a range of 10-30 millimicrons is preferred, whereas for silica and metal particles the individual particle size should preferably be within the range 1-5 micron. Particles of metals, for example stainless steel, exhibit a tendency to aggregation and may be used in aggregation form, up to 200 microns in size, or even more, and selected ranges of aggregates may show improved behaviour compared with the rest, according to the nature of the dope solvent and the concentration of the added particles. While the coarser fractions of aggregated metal particles may exercise a greater effect, they may alternatively lead to membrane defect.
The particles themselves exhibit no permeability and when slurried in water they should give a pH of 3-7.
The membranes of the invention may be prepared from a variety of polymers. These are preferably cellulose-based, preferably lower esters or ethers, eg acetate, propionate and butyrate, or methyl, ethyl or propyl cellulose. Other polymers which may be used to prepare the membrane include poly-ion polymers, prepared by reaction of poly-anions with poly-cations, polyvinyl chloride, polyacrylonitrile, poly-olefins and polyacrylic esters, particularly of lower alcohols. Apart from the addition of the inorganic particles, the membranes of the invention may be prepared by methods which are conventional for the preparation of ultrafiltration membranes. Thus, the casting solution after the addition and distribution therein of the inorganic particles, is cast as film in flat, tubular or other convenient form, for example as fibres, preferably at room temperature, but if desired at other temperatures, and is preferably contacted less than a minute afterwards, in a leaching bath, for example water, where the solvent diffuses out through the membrane into the leaching bath while the water from the bath passes through the membrane.
The membrane may also be cast directly onto a porous backing providing adequate mechanical support for the membrane. In any case, preferably the membrane when completed is between 5 and 25 mils in thickness, ie 0.012-0.0625 cms, but membrane thicknesses up to 1 mm or even more may be suitable. Thicker membranes are more robust, but show a corresponding decrease in flux rate. As in conventional procedure in the preparation of semi-permeable membranes, the thickness of the membrane may be controlled by the method of applying the dope to the support on which the film is prepared, and its concentration.
In contrast to membrane cast from volatile solvents, which form an active layer at the air interface that performs the selective filtration function and must be exposed to the solution side of the filtration system for best effect, membranes cast from non-volatile solvents form a corresponding skin serving the same purpose at the interface with the surface on which the film is cast, and this must be exposed with the skin on the filtrate side, remote from the solution undergoing filtration, to exhibit a high flux while being selective to larger molecules in an ultrafiltration capacity. In the preparation of a membrane according to the invention it is found that the greatest flux improvement effected by a given quantity of additive particles occurs when they settle in the membrane casting, concentrating near the interface with the support material in the active layer and thus providing an anisotropic, ie asymmetric distribution. To this end, aggregated particles are preferred which settle rapidly in the dope. Sufficient time should be permitted for this to occur, but the membrane should in any event be leached to remove solvent, within five minutes of completing the casting. An asymmetric distribution may however be encouraged in internal membranes supported on tubes, by rotating these to apply centrifugal force to the particles in the casting. In use, the prepared membrane is mounted in a suitable test cell or similar arrangement providing adequate mechanical support for the membrane and the milk or other liquid system to be filtered is supplied under pressure to the contact surface of the membrane. The liquid is usually circulated continuously until the degree of concentration required is obtained.
In the following Examples a series of membranes was prepared using a variety of particulate additive materials, all of which exhibited a specific surface area of at least 30.sup.2 m/gm and a pH of about 5. This was measured by immersing an electrode of a pH meter in the supernatant liquor obtained by slurrying about 5% of the material under test in water.
The stainless steel particles used were of 316L stainless steel, containing 14% nickel, 17% chromium and 2.5% molybdenum. They were nominally 5 microns diameter but aggregated. In Examples 5 and 6 the particles were sieve-graded and the fractions obtained were used in separate tests to demonstrate the effect of the extent of aggregation between the particles upon the flux rate. About three-quarters of the aggregate was of mesh sieve size 60-90 microns.
In each case the same grade of secondary cellulose acetate was used to prepare the membrane, which was cast at about 15.degree. C.